1. Field of the Invention
This invention relates generally to component system flux-gate magnetometers of the tall-toroidal type, and particularly to a single core three axis magnetometer having a magnetic tape core.
2. Description of the Prior Art
The term magnetometer refers to instruments for measuring weak magnetic fields, the upper limit generally being considered the maximum intensity of the earth's magnetic field. Magnetometers are commonly divided into three classifications: mechanical magnetic balances, which compare the magnetic field intensity to a known force (gravity, torsion, etc.); component systems, which measure the effect of the magnetic field intensity or changes in the magnetic field intensity along an axis of a sensor (coils, Hall effect, electron beam); and spin-precession detectors, which monitor the interaction of the magnetic field with energized or moving atomic particles (proton free precession, optical pumping, and monitoring types.) The first two classifications are generally grouped under the broader heading of electronic magnetometers.
Electronic magnetometers measure one vector component of the magnetic field, and are therefore orientation dependent. Relative field intensity is determined by amplitude variation, therefore requiring accurate calibration and tuning for optimal performance. Traditionally, the term magnetometer has referred to a single field sensing unit, with two or three orthogonal sensing units being combined to measure the total magnetic field. Commercial devices which measure more than one orthogonal vector are thus termed two-axis (biaxial) or three-axis (triaxial) multi-component system magnetometers.
In some types of component systems, the sensing devices are fixed relative to the magnetic field vector component being observed. The two most frequently employed examples are flux-gates and search-coils.
A flux-gate consists of a high permeability core in which the ambient field induces a magnetization. A primary or excitation winding around the core is electronically stimulated using alternating current until the core reaches saturation. Secondary, sensing, or output windings measure the asymmetry of the magnetization in the core resulting from the superimposed ambient field, which appears as a second harmonic of the sweep or excitation frequency. The resultant output is most commonly a voltage proportional to the particular vector component's field strength or intensity.
In contrast, search-coils measure variations of the ambient field which induce voltages in the primary winding proportional to the change in field or frequency over time. Since response falls off to zero at low frequencies, these devices function only as variometers.
Flux-gate sensor cores may generally be of an open or closed configuration. Four common types of flux-gate sensor cores have well defined analytical characteristics: single rod, double rod, toroid or ring core, and tall-toroidal core. (The term tall-toroid is used herein to describe a hollow cylindrical tube or annulus.)
The single and double rod cores are examples of open cores, in which rods, strips, or wires of high permeability material are wound with excitation and output windings, with single windings being used in some applications for both excitation and output as in the case of search-coils. The second harmonic of the output voltage is obtained by filtering. In double rod core systems, the rods are arranged in opposition (i.e., two parallel spaced-apart rods having the direction of the excitation windings and induced field opposing one another) with the output windings surrounding both rods generally perpendicular to their lengths.
In toroid or ring cores, the core is generally formed from a curved rod or disk of malleable high permeability material having a circular or square half-annular cross section, and may have a circular or elliptical overall shape. The excitation windings are wound toroidally, while the output windings are wound either toroidally or circumferentially (axially) across the diameter of the ring core. In tall-toroidal cores, the core material is molded or machined to the desired dimensions in the shape of a hollow cylindrical tube or annulus, with the core having a generally rectangular half-annular cross section with a height greater than the radial thickness. The excitation windings are again wound toroidally, and the output windings may be wound equatorially rather than axially.
Iron and various high permeability materials (mu metal coated glass rods, Supermalloy,.TM. Permalloy 80.TM.) have proven suitable for forming many types of flux-gate cores. One improvement has been the use of a very high permeability magnetic tape material in single rod cores. U.S. Pat. No. 4,851,775 to Kim describes a very small single rod core using a straight strip of magnetic tape material (Metglas.TM. amorphous alloy 2705M) having a length dimension of 1.8 cm, width dimension of 0.5 mm, and a thickness of 20 .mu.m. This type of a magnetic tape material provides advantages in fabricating such a core due to the methods by which the material can be handled and worked.
While magnetic tape materials have proven preferable for single and double rod cores, a significant disadvantage remains in attempting to form a tall-toroidal core from a planar magnetic tape material--namely, the ends of the magnetic tape strip form discontinuities that "leak" the induced magnetic field orthogonal to the preferred orientation of the induced field, thereby overlapping the ambient field along at least one field vector component and creating unwanted output signal noise. Filtering this signal noise requires more complicated circuitry and greatly increases the cost of both single- or multi-component systems. While magnetic tape materials may be utilized in single-axis component systems where the output windings may be oriented parallel with the leakage of the induced field, the magnetic tape materials have proven less than optimal for multi-axis sensors -where the leakage must necessarily affect sensing along at least one vector component.
Various methods of producing a multi-component system incorporating biaxial or triaxial sensors are known. Conventional triaxial sensors are produced using a first biaxial ring core sensor having two orthogonal axial output windings to measure two field vector components, in combination with a second single-axis ring core having one axial output winding oriented orthogonal to both vector components of the biaxial ring core sensor and spaced apart therefrom by approximately one half the core diameter. Such a prior art triaxial multi-component system is shown in FIG. 10 herein. The left sensor core includes two axial or circumferential output windings which measure field vector components relating to the marked Y- and Z-axes, while the right sensor core includes a single axial or circumferential output winding oriented orthogonal to both the output windings of the left sensor core to measure the remaining field vector component relating to the marked X-axis.
Dual core systems are suitable for many terrestrial or navigational uses where the source of the ambient field is located at a distance sufficient that the parallax between the two cores may be neglected, or wherein significant heading error may be tolerated or corrected. However, such a system is unsuitable for observing near-field events where the directional vector itself rather than the magnitude of the ambient magnetic field is critical, and the angle formed between the ambient field source and the spaced-apart cores would lead to noticeable error in measurement of those field vector components. One example would be monitoring spatial orientation or telemetry of a movable object relative to a fixed alternating current reference field near or within an enclosed space, such as an aircraft or orbital vehicle. Moreover, since the cores do not saturate in exact synchronization, the "late" core will distort the surrounding field and can therefore produce significant heading errors in navigational systems.
One representative example of a three-axis sensor designed to overcome the problems of vector misalignment and asynchronization is shown in U.S. Pat. No. 4,462,165 to Lewis, which is actually a closed core system that operates on the principle of three orthogonal sets of double rod cores. While it may be appreciated that the Lewis '165 triaxial sensor may be constructed to occupy a relatively small volume with paired output windings of the three sets of spaced-apart rod cores producing a single effective centerpoint for the sensor, the fabrication of the core structure and application of the excitation and output windings are very complicated, labor intensive, and expensive processes. Moreover, fabrication of the core structure to meet uniform tolerances and specifications can be very difficult.